Halley's Comet III

StarDate: May 13, 2010

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76 years can make quite a difference. When Comet Halley swung close to Earth a century ago this month, for example, the big story was that its tail contained a deadly gas -- a story that had many people holing up until the comet passed. But the next time it flew past Earth, in 1986, the big story was that a European spacecraft was flying through Halley's tail.

In fact, Halley's 1986 appearance was the most intensively studied appearance of any comet in history -- before or since. Astronomers around the world watched it closely for several years.

As Halley drew close to the Sun, some of its icy surface evaporated, releasing gas and dust. This material formed a big cloud around the comet's nucleus, plus a long, glowing tail.

Two Soviet probes flew through the outer fringes of the tail. An American spacecraft watched the comet from orbit around Venus, while instruments aboard a space shuttle watched from orbit around Earth.

The star of the show, however, was a European craft named Giotto. It snapped the first close-up pictures of the comet, showing that its surface is almost as black as tar. It found that Halley was a good bit bigger than expected, and it mapped "jets" of ice and dust spewing into space. It also measured the composition of the material around the comet -- mostly water vapor, with a smattering of other compounds. Giotto got a little battered, but it survived its encounter with Halley's worrisome tail.

Script by Damond Benningfield, Copyright 2010

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